1. Field of the Invention
The invention is related to the field of magnetic disk drive systems and, in particular, to magnetic recording heads having electrostatic discharge (ESD) shunt traces formed from magnetoresistive (MR) layers. More particularly, the ESD shunt traces formed from MR layers are processed to reduce the MR properties of the ESD shunt trace.
2. Statement of the Problem
Many computer systems use magnetic disk drives for mass storage of information. Magnetic disk drives typically include one or more magnetic recording heads (sometimes referred to as sliders) that include read elements and write elements. A suspension arm holds the recording head above a magnetic disk. When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) side of the recording head to ride a particular height above the magnetic disk. The height depends on the shape of the ABS. As the recording head rides on the air bearing, an actuator moves an actuator arm that is connected to the suspension arm to position the read element and the write element over selected tracks of the magnetic disk.
To read data from the magnetic disk, transitions on a track of the magnetic disk create magnetic fields. As the read element passes over the transitions, the magnetic fields of the transitions modulate the resistance of the read element. The change in resistance of the read element is detected by passing a sense current through the read element and then measuring the change in voltage across the read element. The resulting signal is used to recover the data encoded on the track of the magnetic disk.
The most common type of read elements are magnetoresistive (MR) read elements. One type of MR read element is a Giant MR (GMR) read element. GMR read elements using two layers of ferromagnetic material (e.g., NiFe) separated by a layer of nonmagnetic material (e.g., Cu) are generally referred to as spin valve (SV) elements. A simple-pinned SV read element generally includes an antiferromagnetic (AFM) layer, a first ferromagnetic layer, a spacer layer, and a second ferromagnetic layer. The first ferromagnetic layer (referred to as the pinned layer) has its magnetization typically fixed (pinned) by exchange coupling with the AFM layer (referred to as the pinning layer). The pinning layer generally fixes the magnetic moment of the pinned layer perpendicular to the ABS of the recording head. The magnetization of the second ferromagnetic layer, referred to as a free layer, is not fixed and is free to rotate in response to the magnetic field from the magnetic disk. The magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS in response to positive and negative magnetic fields from the rotating magnetic disk. The free layer is separated from the pinned layer by the spacer layer, which is nonmagnetic and electrically conducting.
Another type of spin valve read element is an antiparallel pinned (AP) SV read element. The AP-pinned SV read element differs from the simple pinned SV read element in that an AP-pinned structure has multiple thin film layers forming the pinned layer instead of a single pinned layer. The AP-pinned structure has an antiparallel coupling (APC) layer between first and second ferromagnetic pinned layers. The first pinned layer has a magnetization oriented in a first direction perpendicular to the ABS by exchange coupling with the AFM pinning layer. The second pinned layer is antiparallel exchange coupled with the first pinned layer because of the selected thickness of the APC layer between the first and second pinned layers. Accordingly, the magnetization of the second pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetization of the first pinned layer.
Another type of MR read element is a Magnetic Tunnel Junction (MTJ) read element. The MTJ read element comprises first and second ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ read element, the first ferromagnetic layer has its magnetic moment pinned (referred to as the pinned layer). The second ferromagnetic layer has its magnetic moment free to rotate in response to an external magnetic field from the magnetic disk (referred to as the free layer). When a sense current is applied, the resistance of the MTJ read element is a function of the tunneling current across the insulating layer between the ferromagnetic layers. The tunneling current flows perpendicularly through the tunnel barrier layer, and depends on the relative magnetization directions of the two ferromagnetic layers. A change of direction of magnetization of the free layer causes a change in resistance of the MTJ read element, which is reflected in voltage across the MTJ read element.
GMR read elements and MTJ read elements may be current in plane (CIP) read elements or current perpendicular to the planes (CPP) read elements. Read elements have first and second leads for conducting a sense current through the read element. If the sense current is applied parallel to the major planes of the layers of the read element, then the read element is termed a CIP read element. If the sense current is applied perpendicular to the major planes of the layers of the read element, then the read element is termed a CPP read element.
An MR read element in a recording head is highly susceptible to damage from electrostatic discharge (ESD) during fabrication, during handling, and during use in the field. The handling and use of recording heads may result in a buildup of electrostatic charges on the various elements of the recording head or other objects contacting the recording head. As an example, electrostatic charges may be built up at various steps during wafer processing. When an MR read element is exposed to ESD, or even a voltage or current input larger than that intended under normal operating conditions, referred to as electrical overstress (EOS), the MR read element may be damaged.
One solution to the problem is to connect ESD protection devices to the MR read element. For instance, a resistor may be connected to one or both of the shields proximate to the MR read element to dissipate electrical potentials that can damage the MR read element. The resistors can be made from the same MR layers as the MR read element in the same fabrication steps. Semiconductor diodes can alternatively be connected to the MR read element and to other structures in the recording head, such as the shields, the substrate, or to the other terminal of the MR read element. The diodes can be connected to the read head using conductors that are made from the same MR layers as the MR read element in the same fabrication steps.
Unfortunately, when ESD protection devices or the conductors wiring such devices are fabricated from the same MR layers as the MR read element, stray magnetic fields can switch the resistance state of the ESD protection device. The change in resistance state of the ESD protection device can affect the resistance state of the MR read element, which can interfere with testing, fabrication, and operation of the recording head.
To protect the ESD protection devices from stray magnetic fields, the shields in the recording heads were formed around the ESD protection devices. One drawback to this solution is that it places a lower limit on the size of the shields in the recording heads. It may also limit the ability to lower the overall capacitance of the recording head, which can affect the frequency response characteristics of the recording head. It may also make the recording head more prone to dead shorts between the shield and the MR read element.
It would be desirable to fabricate recording heads with more effective ESD protection devices than those currently proposed.